The invention relates to a method for measuring the centricity of a conductor in an insulating casing, in which the string formed by the conductor with its insulating casing is moved along a conveying direction. Moreover, the invention relates to a corresponding device.
Measurement methods are known in which the position of the conductor is measured with an inductive measuring apparatus and the position of the insulating casing surrounding the conductor with an optical measuring apparatus in a common measuring plane. Based on both measurements, the centricity of the conductor in the insulating casing is determined This type of embodiment is problematic in that the measuring apparatuses can get in each other ways due to their arrangement in the same measuring plane. This circumstance is amplified when, as is often required, two optical measuring apparatuses are used.
The so-called WANDEXE eccentricity measuring apparatus of the applicant with two optical diameter testing devices, which are arranged offset with respect to each other in a measuring head by 90°, and an inductive measuring circuit arranged in the same measuring plane as the optical diameter measuring apparatuses is known from DE 25 177 09 C3. A contact-free system and method for measuring the centricity and the diameter of a string are known from EP 1 495 284 B1. The position of the string is thereby measured in an optical measuring plane by means of an optical measuring apparatus. In a first inductive measuring plane, which lies in the conveying direction of the string in front of the optical measuring plane, the position of the conductor is measured by means of an inductive measuring apparatus. In a second inductive measuring plane, which lies in the conveying direction of the string behind the optical measuring plane, the position of the conductor is also measured with a second inductive measuring apparatus. The inductively measured positions of the conductor are correlated such that one position of the conductor in the optical measuring plane results. Among other things, the centricity of the conductor in the insulating casing is determined from this determined position of the conductor and the measured position of the line in the optical measuring plane.
In the case of the known systems and methods, measurement errors result in the case of a curvature in the string, frequently caused by the sagging of a conductor. In the case of such a curvature, the position of the conductor derived from the inductive measurements in the optical measuring plane does not match the actual position of the conductor in the optical measuring plane. If the string has its maximum curvature in the area of the optical measuring plane, the inductive measuring apparatuses measure the same positions of the conductor. However, due to the curvature of the string, these do not correspond with the position of the conductor in the optical measuring plane. Such errors also result when the curvature of the string has its maximum not in the area of the optical measuring plane, but offset with respect to it in the conveying direction. In this case, different positions of the conductor are determined through the inductive measuring apparatuses. However, also in this case, a derived position of the conductor in the optical measuring plane does not correspond with the actual position of the conductor due to the (asymmetrical) curvature. The determined centricity is also faulty due to the erroneously determined positions of the conductor in the optical measuring plane.
The measurement error depends on the curvature radius of the string. For example, a curvature radius of for example 10 m can occur at a distance of 2 m between the support rollers carrying the string. Measurement errors are hereby causes in the area of 20 μm. An equalization of the string curvature through a larger number of support rollers is not possible in practice. Rather, a finally wave-like progression of the string and thus an undefined measurement situation, which leads to an incalculable error, then results. The curvature of the string can also not be avoided by arbitrarily increasing the tractive force exerted on the string. Thus, the string and in particular its conductor already reach the yield point before a complete elimination of the curvature through stretching.
Based on the explained state of the art, the object of the invention is to provide a method and a device of the initially named type, with which the centricity of the conductor in the insulating casing can be reliably determined even in the case of a curvature of the string.